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Two Parts – LLC and MAC

LLC – Logical Link Control - Upward Software Functions (Communications)
MAC – Media Access Control - Lower Software Functions (Lan Functions)
BIA – Unique Burnt In Address 48 bits
24 bits vendor / 24 bits serial number MAC  00:0E:35:1A:E2:99

00:0E:35:           1A:E2:99

All "F's" = BROADCAST

Vendor Lookup Link = http://standards.ieee.org/regauth/oui/index.shtml

Ethernet:
CSMA/CD – Carrier Sense Multiple Access / Collision  Detect     -  Bus Architecture
Each station on a segment sees all the frames, if destination MAC matches the interface the frame is copied into the station. Upon Collision the stations will stop transmitting and to send the Jamming signal.
 

Cisco Link State - Transmits Keep alives every 10 seconds and listens for traffic.

Fields- 802.3 (note 802.2 will encapsulate in this format)
Preamble | 7 Bytes – Gives synchronization
SFD | 1 Byte – Start Frame Delimiter (placed in Preamble)
Destination | 2-6 Bytes – Gives physical address of the interface to receive
Source | 2-6 Bytes – Gives physical address of the sending interface
LEN Data unit in Bytes
Type  | 2 Bytes – 0800 TCP/IP –   8138   IPX –   80D5   IBM  SNA > 802.2 only
DATA |  46- 1500 Bytes –
Short frames are RUNTS < 64 bytes
Long frames are Jabbers > 1500
PAD 1500 total max
FCS 4 Bytes
 

Most frames are 64 bytes in length, with 30 bytes of headers.

Wire Guides
10base 2 - Thin Coax, 200 Meters, RG-58 connector
10base 5 - Thick Coax, 500 Meters, RG-58 connector
10base T – Star topology, 100 meters, RJ-45 connector pins 1,2,3 and 6
10base F – Multimode Fiber, 2000 meters, ST connector
 

Token Ring
Two unidirectional paths (TX and RX), need token to transmit new data.
Traffic flows from station to station in ring order.
If matching mac address – Take Frame, send data to buffers.
If no match – Regenerate Frame on wire, CRC/headers/data.

Token Claiming – Active Monitor process, runs the ring.

Frame Format
SD 1 Bytes – Start Frame Delimiter
AC 1 Byte – Access Control
FC 1 Byte – Frame Control
Destination 6 Bytes – Gives physical address of the interface to receive
Source 6 Bytes – Gives physical address of the sending interface
RI N Bytes – Routing Indicator (only used when leaving local segment) RIF
IF N Bytes – Information Field - Data up to max MTU (4096)
FCS 4 Bytes - Frame Check
ED 1 Byte – End Frame Delimiter
FS 1 Byte – Frame Status – Frame copied and return Token to ring
 
 
 

Bridging:
Data link layer 2
Transparent –  All seen on one Bridge. No routing bit, monitors / records MAC frame, passes segments.
Translational - Converts Frame Type. Ethernet to token ring, Fddi to Ethernet.
Source Route Bridging – IBM’s method – Assigned ring numbers on segments. Uses EXPLORERS, route is determined before any user data sent.
Source Route Transparent - Combines Source Routing and Transparent Bridging.  Uses RII (Routing Information Indicator) and RIF (Routing Information Field) to move data.

Switches –
Most bridges are learning or adaptive because they will record physical addresses as they pass frames. This will speed next packet delivery and reduce propagation broadcast passing flow flow. Pin points / maps topology.

Store and Forward – Copies entire frame into buffer before sending to destination.
Great for error checking, latency is determined by frame size.
Cut Thru – Starts sending to destination immediately on MAC match. lowest latency.
Fast but no guarantee.
Modified Cut Thru – Start sending immediately after 64 bytes on match and copy to buffer.
Middle road implementation.

Backplane – Fabric

VLAN-

VLAN switching is accomplished through frame tagging, Each VLAN is differentiated by a color, or VLAN identifier. Tag is applied on trunks leaving switch.

     See More on VLANS and Spanning Tree

Spanning Tree – Can have multiple physical routes, with only ONE active.

Spanning Tree Port States:
Disabled – not participating
Blocking – up not passing data
Listening/Learning – Involved
Forwarding – Passing data

Communicates using BPDU – Bridge Protocol Data Units, maps and build topology.
 

Layer two networking protocols:
SNAP  Sub Network Access Point - Works with LLC to identify frames.
NetBIOS is a session layer protocol. Needs to be on same lan segment (bridge), broadcast intensive
NetBEUI is a transport mechanism, Combines transport (4), network (3) and some data link layers (2).

 

No IP Address is carried, all layer two MAC format.
You can tunnel traffic or carry NetBIOS over TCP/IP (Microsoft) and IPX (Novell)

Layer 2 Networking
Token Ring Configuration Example:
interface TokenRing1
ip address 198.83.16.1 255.255.255.0
no ip route-cache
hold-queue 200 in
hold-queue 100 out
ipx network BB7
ring-speed 16
multiring all
source-bridge 34 1 555
source-bridge spanning

Source-route bridging technology is a combination of bridging and routing functions. A source-route bridge is allowed to make routing decisions based upon the contents of the Medium Access Control (MAC) frame header. Keeping the routing function at the MAC or Level 2 layer allows the higher layer protocols to execute their tasks more efficiently, and also allows the LAN to be expanded without the knowledge of the higher layer protocols.

Multiring allows for per protocol specification of the interface's ability to append RIFs to routed protocols. When it is enabled for a protocol, the router will source packets that include information used by source-route bridges. This allows a Cisco router with Token Ring interfaces, for the protocol or protocols specified, to connect to a source-bridged Token Ring network. If a protocol is not specified for multiring, the Cisco router can only route packets to nodes directly connected to its local Token Ring. "ALL: all Protocols".

Cisco Bridge Config

ip subnet-zero
no ip routing
!
!
!
interface Fddi0
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
keepalive 1
bridge-group 1
!
interface FastEthernet0
no ip address
no ip directed-broadcast
no ip route-cache
no ip mroute-cache
keepalive 1
bridge-group 1
!
ip classless
!
!
bridge 1 protocol ieee

Commands

Look for different interfaces and increasing counts.

15NetBridge#show bridge

Total of 300 station blocks, 290 free
Codes: P - permanent, S - self

Bridge Group 1:


    Address       Action   Interface       Age   RX count   TX count
0008.02b3.e6e8   forward   FastEthernet0    0           1          0
0008.02a4.0456   forward   FastEthernet0    0           1          0
0030.7b90.d3b4   forward   FastEthernet0    0         347          0
00d0.d336.84f4   forward   FastEthernet0    0           7          0
00d0.c0d2.93ec   forward   FastEthernet0    0           1          0
0008.c7e9.8411   forward   FastEthernet0    0           1          0
00e0.1893.c758   forward   FastEthernet0    0           1          0
0000.0c07.ac0e   forward   FastEthernet0    0           7          0
00e0.1875.7cd9   forward   FastEthernet0    0           1          0
0002.a53f.59ea   forward   FastEthernet0    0           1          0
0008.c7d8.ec3d   forward   FastEthernet0    0           6          0
0000.650a.588c   forward   Fddi0            0           1          0
00d0.bcf6.fc14   forward   FastEthernet0    0           3          0

15NetBridge#^@

source-bridge source-ring-numberbridge-numbertarget-ring-number [conserve-ring]
Examples

In the following example, Token Rings 129 and 130 are connected via a router:

interface tokenring 0
source-bridge 129 1 130
!
interface tokenring 1
source-bridge active 130 1 129

 To See More On IBM SNA